In the construction of integrated circuit devices, one or more conducting layers (e.g. aluminum, copper, titanium, silicon or various alloys thereof) are deposited and subsequently pattern masked, then etched to create ohmic or Schottky contacts and electrical connections between various circuit elements. Conventionally, thin films of conducting materials are formed; these films are then coated with a spin coated photoresist layer. The photoresist layer is exposed to a light pattern and then developed to form a photoresist mask pattern. Selective etching removes portions of the underlying material through the openings in the photoresist pattern. For example, a metallic aluminum conducting layer would be selectively plasma-etched with chlorine-containing gases. The photoresist is then removed leaving a pattern in the conducting layer.
In the art, when the conducting layers are made of reflective materials (e.g. metallic materials), antireflective coatings (ARC's) have been applied to reduce surface reflection. Typical surfaces benefitting from ARC's are layers formed of polysilicon, aluminum, copper, titanium, or other reflective metals and their alloys. ARC's improve photoresist patterning control by reducing standing wave effects or diffuse scattering caused by reflection of radiation off reflective surfaces. These problems are magnified when monochromatic illumination sources are used. Furthermore, as microcircuit density increases, scattering, diffraction, and interference effects become less and less tolerable. As circuit density increases and feature size and linewidths decrease below the 0.18 .mu.m level, such effects become increasingly critical. As linewidths approach the 0.20 .mu.m level, deep ultraviolet (DUV) exposure sources are commonly used to expose photoresists and provide the necessary definition in mask patterns. DUV radiation can be loosely defined as radiation between the wavelengths of 4-400 nanometers (nm). Typical sources of such radiation are, for example, cadmium, xenon, or mercury lamps, and certain types of excimer lasers. DUV sources are used because it is critically important to produce sharply defined mask patterns. To this end, reflected light must be reduced in order to maximize photoresist pattern definition.
Prior to the use of DUV exposure sources and DUV photoresists, a metallization layer (typically formed of aluminum) was coated with an antireflective layer of titanium nitride (TiN) followed by a spin coating of photoresist. This photoresist was then patterned and subsequently etched, then the photoresist was removed. The TiN layer remained in place to help prevent electromigration and serve as a shunt layer permitting continuous current flow, in the event of void formation in the metallization layer. However, due to the increased resolution possible with DUV exposure sources and the need for smaller and smaller feature sizes, the art has moved towards DUV exposure sources and photoresists optimized to take advantage of DUV sources. Unfortunately, TiN ARC's undergo chemical reactions with DUV photoresists and, therefore, are not compatible with the new photoresists. There is a need for new solutions to the old problem of metallization reflectance.
The well understood and commonly used process procedures previously used are not compatible with the use of DUV exposure sources. New photoresists which are sensitive to DUV light have come into use. However, these DUV photoresists present new problems. The new photoresists are chemically-amplified resists, which means that through chemical treatment, a chemically-amplified resist is more sensitive to light than its non-amplified predecessors. Chemically-amplified resists require less exposure time. For example, standard I-line (365 nm) lithographic exposure requires 200 millijoules (mJ) of activation energy to develop a standard photoresist. In comparison, a typical chemically-amplified resist may only require 10 mJ (at 248 nm). Such typical chemically-amplified resists are manufactured under the tradenames of Apex or UV05, both made by Shipley Company of Marlborough, Mass. However, these new photoresists react to nitrogen containing compounds. Consequently, these photoresists react with the previously used TiN or silicon nitride antireflective layers as well as ambient nitrogen in typical reaction chambers.
FIGS. 1A-1C show the unhappy effect of using DUV photoresists in conjunction with a TiN ARC. FIG. 1A is a cross-section view of a portion of an integrated circuit structure identified generally as 10, having a semiconductor substrate 101, with a reflective metallization layer 102, and a TiN ARC 103, and a DUV photoresist mask pattern 104. As shown in FIG. 1B, the exposed portions of the DUV photoresist 104T react with the nitrogen containing ambient and also with the nitrogen containing TiN ARC layer 103 in region 104B. The effects of this exposure to nitrogen degrade the photoresist as shown in FIG. 1C, creating the irregularly shaped and undesirable photoresist profiles 104E.
FIGS. 1D-1E are plan views of a semiconductor surface which further illustrate some of the problems associated with DUV photoresists and nitrogen-containing ARC's. FIG. 1D shows an idealized structure on a semiconductor surface 101, having microcircuit components (A, B, C, & D) formed thereon. Elements A & B are electrically connected using an idealized metal interconnect 102.sub.AB. Elements C & D are also electrically connected using an idealized metal interconnect 102.sub.CD. The sharply defined features of the metal interconnects 102.sub.AB & 102.sub.CD are desired. Unfortunately, due to the photoresist degradation disclosed above, interconnect problems arise. For example, as in FIG. 1E, the microcircuit components (A, B, C, & D) are electrically connected by the less than ideal interconnects 102.sub.AB ' & 102.sub.CD '. A common effect is interconnect shorting, as shown by S, where separate metallization layers contact each other.
Despite this drawback, DUV photolithography has come in to ever more frequent use due to the increased definition possible with DUV lithography. A great need has arisen for ARC materials that are compatible with the newer photoresists.
One solution is to provide a sacrificial layer of oxide (e.g. silicon dioxide (SiO.sub.2)) over the TiN ARC layer, followed by the application of a DUV photoresist. The sacrificial layer provides a barrier between the TiN ARC and the DUV photoresist. The photoresist is then patterned and fabrication continues as is needed to create the necessary circuit structures. The problem with the oxide sacrificial layer is that it adds an additional process step to each metallization layer. It also requires a separate machine for creating such layers. This drives up cost and increases manufacturing time, in addition to increasing the complexity of the process. Further, by forming an oxide sacrificial layer this process increases the possibility of harmful particle formation during fabrication.
Others have postulated the use of a TiN/Ti ARC bi-layer. The TiN layer is fabricated over the metallization layer, followed by a layer of metallic titanium (Ti) formed over the TiN layer. The top Ti layer prevents a subsequently formed DUV photoresist layer from contacting the TiN layer, thereby preventing the DUV photoresist from reacting with the nitrogen in the TiN layer. Unfortunately, this ARC has the same drawbacks as the sacrificial oxide layer process, namely it is a two step, two chamber process. Furthermore, in a conventional process, the TiN deposition chamber requires Ti pasting to prevent TiN from flaking off the interior surfaces of the chamber, which decreases throughput.
Unfortunately, it has also been observed that with aluminum (Al) metallization layers, a highly resistive Al--N layer may be formed at the interface between the Al metallization layer and a TiN ARC. A solution to this problem is set forth in U.S. Pat. No. 5,582,881 by Besser, et al. which uses a Ti layer to form a barrier between the Al metallization layer and a TiN ARC layer. Again, this does not solve the problem of DUV photoresist reactivity with the TiN top layer.
In U.S. Pat. No. 5,231,053 by Bost, et al. a TiN/Ti/TiN tri-layer is used as a coating to prevent a reaction of underlying aluminum with ambient oxygen and fluorine containing etchants. Unfortunately, the upper TiN layer negatively reacts with a subsequently applied DUV photoresist layer. The invention of Bost et al. also has a TiN top layer which contacts the overlying DUV photoresist and therefore does not solve the problem of nitrogen reacting with DUV photoresist. Further, unlike the present invention, the process of Bost, et al. disadvantageously requires an additional process step to return a Ti target to an un-nitrided state.
In U.S. Pat. No. 5,738,917 ('917), Besser, et al. used a Ti/TiN/Ti layer as an underlayer for a subsequently fabricated Al metallization layer. This structure is used to reduce electromigration of Al into underlying substrates, particularly silicon. Furthermore, the Ti/TiN/Ti layer of '917 is covered with metal rather than photoresist, as in the present invention. Thus, the Ti/TiN/Ti stack of '917 does not protect an overlying DUV photoresist layer from the harmful effects of TiN nor does it serve as an ARC to enhance metallization etching. In fact, '917 uses a TiN ARC which causes exactly the type of problem the present invention seeks to remedy (i.e. preventing DUV photoresist reaction with the TiN ARC). Furthermore, the Ti/TiN/Ti stack structure of '917 may be easily incorporated as a supplementary underlayer in conjunction with the ARC of the present invention.
In U.S. Pat. No. 5,317,187 ('187), Hindman, et al. used a Ti/TiN/Ti underlayer beneath a subsequently fabricated Al metallization layer. This structure is formed above an underlying silicon or passivation layer. The Al metallization layer is then overlaid with a single layer TiN ARC, unlike the three layer ARC stack of the present invention. The purpose of the Ti/TiN/Ti layer of '187 is to react the bottommost Ti layer with the underlying silicon to form a TiSi.sub.2 contact metallization layer. Thus, the Ti/TiN/Ti stack of '187 is not formed over a metallization layer, but rather under it. In fact, the Ti/TiN/Ti stack of '187 must be formed on top of a silicon layer. The absence of a silicon layer prevents the formation of a silicide layer, which defeats the purpose of '187. In being underneath the metallization layer, the Ti/TiN/Ti stack of '187 cannot protect an overlying DUV photoresist layer from the harmful effects of TiN nor can it serve as an ARC to enhance metallization etching. In fact, by placing the aluminum metallization layer on top of the Ti/TiN/Ti stack, the structure of '187 exacerbates the metallization reflectivity problem the present invention seeks to resolve. Finally, the TiN ARC structure of the '187 patent is incompatible with DUV photoresists.
In view of the above, there is a need for overcoming the difficulties and drawbacks of presently used ARC's in conjunction with DUV photoresists. In particular, there is a need for an effective and easily manufacturable ARC which does not chemically react with DUV photoresists or metallization layers. An object of the present invention is to minimize light reflection in a photoresist from underlying reflective surfaces during photolithographic exposure of the photoresist. It is a further object of the present invention to provide an effective ARC layer which does not chemically react with either the underlying metallization layer or undergo chemical reaction with DUV photoresists. Another object of the invention is to provide an effective ARC which may be formed using only one chamber of a multichamber physical vapor deposition (PVD) system. Another object of the present invention is to provide an improved process for producing integrated circuit structures.